The constant demand for increased operating temperatures in gas turbine engines was initially met by air cooling of the turbine blades and development of superalloys from which to manufacture the blades, both of which extended their service lives. Further temperature increases necessitated the development of ceramic coating materials with which to insulate the turbine blades from the heat contained in the gases discharged from the combustion chamber and again turbine operating life was extended. However, the amount of life extension was limited because the coatings suffered from inadequate adhesion to the superalloy substrates, one reason for this being the disparity of coefficients of thermal expansion between the superalloy substrate and the ceramic coating. Coating adhesion was improved by the development of various types of aluminum-containing alloy bond coats which were thermally sprayed or otherwise applied to the superalloy substrate before the application of the ceramic coating. Such bond coats are typically of the so-called aluminide (diffusion) or "MCrAlY" types, where M signifies one or more of cobalt, nickel and iron.
Use of bond coats has been successful in preventing extensive spallation of thermal barrier coatings during service, but localized spallation of the ceramic still occurs where the adhesion fails between the bond coat and the ceramic layer. This exposes the bond coat to the full heat of the combustion gases, leading to premature failure of the turbine blade.
The bond coats of the aluminide (diffusion) type are disclosed for example in U.S. Pat. Nos. 4,880,614, 4,916,022 and 5,015,502. This type of bond coat is produced by reacting aluminum with the superalloy substrate to produce a diffusion aluminide bond coat. The aluminum is reacted with the superalloy substrate by any of the commercially available aluminizing processes using aluminum vapors or aluminum rich alloy powder, for example pack aluminizing, chemical vapor deposition, sputtering, electrophoresis, etc. and is followed by diffusion heat treatment. These patents also disclose the use of platinum aluminide bond coats on the superalloy substrate.
The bond coats of the MCrAlY type are disclosed in the U.S. Pat. Nos. 4,321,311, 4,401,697 and 4,405,659. This type of bond coat is produced by depositing a MCrAlY alloy onto the superalloy substrate.
Furthermore the three U.S. Pat. Nos. 4,880,614, 4,916,022 and 5,015,502 mentioned above also disclose the use of an aluminide coating in conjunction with a MCrAlY coating as a bond coating. More specifically they disclose that the substrate is first aluminized, as discussed above, and then a MCrAlY coating is applied onto the aluminized superalloy substrate.
Also International Patent application No. WO93/18199 discloses the use of an aluminide coating in conjunction with a MCrAlY coating as a bond coating. More specifically it discloses that the superalloy has a MCrAlY coating with an aluminide top coating or a MCrAlY coating with a platinum aluminide top coating.
It is further known from U.S. Pat. No. 4,399,199 to provide a platinum-group metal layer on a superalloy substrate as a bond coating for a ceramic thermal barrier coating. The platinum-group metal is heat treated at 700.degree. C. to bond the platinum-group metal to the superalloy substrate.
It is also known from U.S. Pat. No. 5,427,866 to provide a platinum-group metal layer on a superalloy substrate as a bond coating for a ceramic thermal barrier coating. The platinum-group metal is heat treated at 980.degree. C. to 1095.degree. C. to form an interdiffusion region of platinum-group metal aluminide between the superalloy substrate and the platinum-group metal.
A problem associated with the production of the platinum aluminide on the superalloy substrate is that the use of the conventional aluminizing process, e.g. pack aluminizing, uses a pack containing aluminum oxide powder and aluminum halide which produces aluminum vapors to react with platinum deposited on the superalloy substrate. This pack also contains undesirable elements, or impurities, which also react with the platinum on the superalloy substrate leading to poor adhesion between the platinum aluminide and the ceramic coating.
A problem associated with the production of the interdiffusion region of platinum aluminide between the platinum and the superalloy substrate is that the bond coating is unstable leading to poor adhesion between the ceramic coating and the bond coating.